Method of sifting fine powder



March 22, 1966 A. E. NEWTON METHOD OF SIFTING FINE POWDER Filed Aug. 20, 1962 w d 4 M as hi my r d aw mm a? M 60 60 70 80 90 I00 //0 I I I I M Q RN 1 W m QJ Md nfu/( F &

i regu n y In venior Alberz F. Newzon By his Azzorney United States Patent 3,241,672 METHOD OF SIFTING FINE POWDER Albert E. Newton, Beverly, Mass., assignor to United Shoe Machinery Corporation, Boston, Mass, 2 corporation of New Jersey Filed Aug. 20, 1962, Ser. No. 217,934 1 Claim. (Cl. 209302) This invention relates to sifting and more particularly to sifting of powders, especially fine powers, for classification or dispensing.

In the common practice of sifting fine powders, exemplified by the use of shaker screens and the like, the caking of the fine powder as the result of vibration has been a problem. The vibrated powder tends to form a dense mass which clogs the apertures of the screen and inhibits proper transfer of the powder therethrough.

Accordingly, it is an object of the present invention to provide a method by which fine powders may be sifted with increased rapidity and efiiciency.

To these ends and in accordance with features of the present invention, there is provided a method of sifting powder which increases the efficacy of sifting and obviate the foregoing problem.

I have found that caking may be overcome and other advantages achieved by regulation of the frequency of vibration of powder supported in a container. In the vibration of fine powders in a screened receptacle there is, in the frequency spectrum, a relatively sharp peak defining a range of frequencies in which a transition occurs between agitation and violent turbulence of the powder in the receptacle. In this range, the powder, rather than clogging and aggregating, becomes highly active and dispersed.

This turbulence effect is most marked in the upper portions of the powder mass within the container, apparently being somewhat damped at the bottom by the weight of powder above. Accordingly, I prefer to employ a powder receptacle comprising a vertical or substantially vertical screened area and to maintain opposite this the area of maximum turbulence so far as possible to effect maximum rate of transfer of the powder through the screen. Some transfer occurs, of course, through other screen areas, for example, at the bottom of the container, but in general in the practice of my invention the substantially vertical areas of the screen adjacent the areas of maximum turbulence are the most important in regard to the maximum rate of sifting.

The rate of transfer of the powder at the frequency of maximum turbulence, F may be from two to three or more times as large as the rate of transfer at frequencies cycles per second greater or smaller than F The above and other features and advantages of the invention will best be understood from the following description taken in connection with the accompanying drawing in which,

FIG. 1 is a front elevation of the apparatus embodying the present invention; and

FIG. 2 shows spectrum curves to which reference is made in the following description.

Referring to the drawing, the illustrative apparatus comprises a base 10 carrying a yoke mount 12 and a loudspeaker driver unit 14 carried on a bracket 16 attached to the base by screws 18 and 20. At the upper portion of the mount 12 is a top rail 22 from which depend two resilient metal strips 24, 26 which are held to the rail at opposite surfaces thereof by screws 28 and 30. A carrying handle 32 is attached to the mount 12 at its top.

The lower ends of the strips 24 and 26 are provided with alined apertures through which passes a rod 34. One end of the rod is connected to a diaphragm 36 of the river 14 by a small rod 38, one end of which is adhesively bonded to the diaphragm 36 while the other end ice is fastened to the rod 34 by a screw 40 threaded therein and to the rod 38.

To the other end of the rod 34, a light metal hopper 42 is connected by a screw 44 threaded into the rod 34. The screws 40 and 44 are held firmly in place by locknuts 46;

A metal ring 48 is fastened to the hopper 42 and to this ring is secured a cylindrical receptacle 50 formed, including its bottom, of wire screen.

Supported on the mount 12 adjacent the base thereof is a combined oscillator-amplifier 52 having a frequency control 54 and amplitude control 56. The output of the amplifier is connected to the driver 14 by a cable 58 so that during operation the unit may be caused to vibrate the container 50 at a predetermined variable frequency and amplitude, to transfer powder from the container to a plastic bag 60.

In a typical operation of .an illustrative embodiment of the present invention, in which the receptacle S0 was a cylinder of 70 mesh screen 1% "in diameter by 2% high, an observation run was made through the frequency spectrum from about 40 c.p.s. to over 200 c.p.s. at a constant input to the driver of 20 watts and with the receptacle three-quarters full of 60-140 mesh thermoplastic powder. It was noted that in the range up to 60 c.p.s., the powder was agitated in the receptacle as would normally be expected, the particles moving in tune with the movement of the shaken receptacle.

At 60 c.p.s. some evidence of turbulence began to ap pear wherein some of the particles began to boil upwardly. This turbulence increased rapidly with the frequency to a maximum at c.p.s. The frequency at which such a maximum occurs is designated F As the frequency was raised above 75 c.p.s. the turbulence decreased progressively until at 90 c.p.s. it disappeared and the particles were agitated in tune with the receptacle as was the case below 60 c.p.s. At 75 c.p.s. the sifting rate was found to be .53 gram per second whereas the rate at 60 c.p.s. was .34 gram per second, and the rate of sifting at 90 c.p.s. was .26 gram per second. The frequency of maximum turbulence, P is influenced in part by the weight of the powder to be sifted. For example, with the screen receptacle three-quarters full with equal volumes of thermoplastic powder and sand, F, was determined to lie at 72 c.p.s., the rate of sifting being .30 gram per second. The results of these and other experiments are graphically illustrated in FIG. 2. Although the peak corresponding to the mixture of powder and sand is somewhat blunter than that of the powder alone, it will be noted that in both cases a few cycles above or below F the rate of sifting is markedly reduced. F, is also affected by the amount of powder in the receptacle. For example, F, is 78 c.p.s. with the receptacle one-half full and 80 c.p.s. with the receptacle one-quarter full. The sifting rates are correspondingly reduced to .28 gram per second and .10 gram per second respectively. The sifting of a three-quarter full receptacle, until about 10% of the original amount remained, gave a residue consisting essentially of 60-70 mesh particles very cleanly separated from the finer powder.

While the illustrative data relates to sifting powder of a particular size range through a screen of a particular mesh, I have found that finer powders may be sifted through correspondingly finer mesh screens with equal efficiency.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

The method of sifting powders which comprises supporting the powder to be sifted within a receptacle comprising a substantially vertically disposed wall portion including a screening area vibrating the receptacle at a fundamental frequency within the range of a few cycles per second more or less than the frequency F effective to produce maximum turbulence in the upper region of the powder in the receptacle, and during the sifting operation feeding the powder to be sifted to the receptacle as required to maintain the area of maximum turbulence adjacent the screening area of the wall portion.

UNITED References Cited by the Examiner STATES PATENTS FOREIGN PATENTS Great Britain. Great Britain. Switzerland.

HARRY B. THORNTON, Primary Examiner.

HERBERT L. MARTIN, Examiner. 

